Structure-activity relationships of novel anti-malarial agents. Part 7: N-(3-benzoyl-4-tolylacetylaminophenyl)-3-(5-aryl-2-furyl)acrylic acid amides with polar moieties

In a previous report, we have provided evidence that novel anti-malarial compounds based on 2,5-diaminobenzophenone farnesyltransferase inhibitors might benefit from the presence of a polar moiety at the para position of the terminal phenyl of the arylfurylacryloyl partial structure. Here, we demonstrate that different moieties with hydrogen bond acceptor properties lead to equipotent or even improved anti-malarial activity in comparison to the nitro group described before.     

Plasmodium falciparum: stage specific effects of a selective inhibitor of lactate dehydrogenase

Plasmodium falciparum lactate dehydrogenase (PfLDH) is essential for ATP generation. Based on structural differences within the active site between P. falciparum and human LDH, we have identified a series of heterocyclic azole-based inhibitors that selectively bind within the PfLDH but not the human LDH (hLDH) active site and showed anti-malarial activity in vitro and in vivo. Here we expand on an azole, OXD1, from this series and found that the anti-P. falciparum activity was retained against a panel of strains independently of their anti-malarial drug sensitivity profile. Trophozoites had relatively higher PfLDH enzyme activity and PfLDH-RNA expression levels than rings and were the most susceptible stages to OXD1 exposure. This is probably linked to their increased energy requirements and consistent with glycolysis being an essential metabolic pathway for parasite survival within the erythrocyte. Further structural elaboration of these azoles could lead to the identification of compounds that target P. falciparum through such a novel mechanism and with more potent anti-malarial activity.     

Structure-activity relationships of novel anti-malarial agents. Part 2: cinnamic acid derivatives

We have described compound 1 as a lead structure for a novel class of anti-malarial agents. Replacement of the 3-phenylpropionyl moiety of the lead structure 1 by a 4-propoxycinnamic acid residue resulted in a significant improvement in antimalarial activity. Compound 3q represents an important step in the development of lead structure 1 into an anti-malarial drug candidate.     

Novel in vivo active anti-malarials based on a hydroxy-ethyl-amine scaffold

A novel series of anti-malarials, based on a hydroxy-ethyl-amine scaffold, initially identified as peptidomimetic protease inhibitors is described. Combination of the hydroxy-ethyl-amine anti-malarial phramacophore with the known Mannich base pharmacophore of amodiaquine (57) resulted in promising in vivo active novel derivatives.     

Anti-malarial activity of new N-acetyl-l-leucyl-l-leucyl-l-norleucinal (ALLN) derivatives against Plasmodium falciparum

Malaria is the most common of the parasitic diseases in tropical and subtropical regions. Adverse side effects of anti-malarial drugs have precluded them as a potential clinical drug. In this study, novel derivatives of N-acetyl-l-leucyl-l-leucyl-l-norleucinal (ALLN) based on a variety of dipeptidyl α,β-unsaturated amides containing lysine as a part were synthesized and evaluated. Lower toxicity was achieved by reducing or eliminating the tendency of forming chemically reactive and toxic intermediates and metabolites. The synthesized compounds were evaluated for anti-malarial efficacy against Plasmodium falciparum and cytotoxicity in human epitheloid carcinoma cervix (HeLa cells) by estimating the therapeutic index (TI). N-Methyl amide with N′-Boc protection among them exhibited strong anti-malarial activity and N-methyl amide with N′-m-methylbenzyl amide showed excellent anti-malarial activity with much lower toxicity than the ALLN. Therefore, the two chemicals, as well as the underlying design rationale, could be useful in the discovery and development of new anti-malarial drugs.     

Synthesis and exploration of novel curcumin analogues as anti-malarial agents

Curcumin, a major yellow pigment and active component of turmeric, has been shown to possess anti-inflammatory and anti-cancer activities. Recent studies have indicated that curcumin inhibits chloroquine-sensitive (CQ-S) and chloroquine-resistant (CQ-R) Plasmodium falciparum growth in culture with an IC(50) of approximately 3.25 microM (MIC=13.2 microM) and IC(50) 4.21 microM (MIC=14.4 microM), respectively. In order to expand their potential as anti-malarials a series of novel curcumin derivatives were synthesized and evaluated for their ability to inhibit P. falciparum growth in culture. Several curcumin analogues examined show more effective inhibition of P. falciparum growth than curcumin. The most potent curcumin compounds 3, 6, and 11 were inhibitory for CQ-S P. falciparum at IC(50) of 0.48, 0.87, 0.92 microM and CQ-R P. falciparum at IC(50) of 0.45 microM, 0.89, 0.75 microM, respectively. Pyrazole analogue of curcumin (3) exhibited sevenfold higher anti-malarial potency against CQ-S and ninefold higher anti-malarial potency against CQ-R. Curcumin analogues described here represent a novel class of highly selective P. falciparum inhibitors and promising candidates for the design of novel anti-malarial agents.     

Structure-activity relationships of novel anti-malarial agents. Part 4: N-(3-benzoyl-4-tolylacetylaminophenyl)-3-(5-aryl-2-furyl)acrylic acid amides

In a previous report, we have described novel anti-malarial compounds based on a 2,5-diaminobenzophenone scaffold. Here, we have invesigated acryloyl derivatives carrying a biaryl structure consisting of a terminal aryl residue and a central 2-furyl ring. Several compounds were obtained in the series of para-substituted phenylfurylacryloyl derivatives that displayed improved anti-malarial activity in comparison to earlier described derivatives. From the structure-activity relationships it can be deduced that there has to be a lipophilic moiety in the para-position of the terminal phenyl residue. Furthermore, there are indications that, alternatively, activity may benefit from the presence of a polar moiety with hydrogen bond acceptor properties.     

Evaluation and lead optimization of anti-malarial acridones

With 2-methoxy-6-chloroacridone as a lead compound, we synthesized and tested acridone derivatives to develop a better understanding of the anti-malarial structure-activity relationships. Over 30 acridone derivatives were synthesized. The most potent compounds contained extended alkyl chains terminated by trifluoromethyl groups and located at the 3-position of the tricyclic system. Acridones optimized in the length of the side chain and the nature of the terminal fluorinated moiety exhibited in vitro anti-malarial IC(50) values in the low nanomolar and picomolar range and were without cytotoxic effects on the proliferation and differentiation of human bone marrow progenitors or mitogen-activated murine lymphocytes at concentrations up to 100,000-fold higher. Based on a structural similarity to known anti-malarial agents it is proposed that the haloalkoxyacridones exert their anti-malarial effects through inhibition of the Plasmodium cytochrome bc(1) complex. Haloalkoxyacridones represent an extraordinarily potent novel class of chemical compounds with the potential for development as therapeutic agents to treat or prevent malaria in humans.     

Structure-activity relationships of novel anti-malarial agents. Part 3: N-(4-acylamino-3-benzoylphenyl)-4-propoxycinnamic acid amides

We have described 5-(4-propoxycinnamoylamino)-2-(4-tolylacetylamino)benzophenone 6e as a novel lead for anti-malarial agents. Anti-malarial activity of these 5-(4-propoxycinnamoylamino)benzophenones proved to be quite sensitive against variations of the acyl substituent at the 2-amino group. Best activity was obtained with phenylacetic acid moieties carrying small substituents in the para-position. From the para-substituents evaluated, the trifluoromethyl group yielded the most active compound (6j) in this series (IC50=120 nM). Deviations from the phenylacetic acid substructure, shifting the substituent into the ortho-position or bulkier para-substituents resulted in a significant reduction in anti-malarial activity.     

Co-treatment with the anti-malarial drugs mefloquine and primaquine highly sensitizes drug-resistant cancer cells by increasing P-gp inhibition

The purpose of this study was to identify conditions that will increase the sensitivity of resistant cancer cells to anti-mitotic drugs. Currently, atovaquine (ATO), chloroquine (CHL), primaquine (PRI), mefloquine (MEF), artesunate (ART), and doxycycline (DOY) are the most commonly used anti-malarial drugs. Herein, we tested whether anti-malarial drugs can sensitize drug-resistant KBV20C cancer cells. None of the six tested anti-malarial drugs was found to better sensitize the drug-resistant cells compared to the sensitive KB cells. With an exception of DOY, all other anti-malarial drugs tested could sensitize both KB and KBV20C cells to a similar extent, suggesting that anti-malarial drugs could be used for sensitive as well as resistant cancer cells.     

Photo-oxygenation of geraniol: synthesis of a novel series of hydroxy-functionalized anti-malarial 1,2,4-trioxanes

Photo-oxygenation of geraniol 2, an abundantly available allylic alcohol, furnished a mixture of mono- and di-hydroperoxy products; the latter have been used for the preparation of a novel series of hydroxy-functionalized anti-malarial 1,2,4-trioxanes (7a-d, 8a-d).     

Antimalarial activity of N(6)-substituted adenosine derivatives. Part 3

A series of novel 3'-amido-3'-deoxy-N(6)-(1-naphthylmethyl)adenosines was synthesized applying a polymer-assisted solution phase (PASP) protocol and was tested for anti-malarial activity versus the Dd2 strain of Plasmodium falciparum. Further, this series and 62 adenosine derivatives were analyzed regarding 1-deoxy-d-xylulose 5-phosphate (DOXP) reductoisomerase inhibition. Biological evaluations revealed that the investigated 3',N(6)-disubstituted adenosine derivatives displayed moderate but significant activity against the P. falciparum parasite in the low-micromolar range. On the molecular level, DOXP reductoisomerase utilizing an adenosyl-containing substrate was identified as a promising metabolic target for ligands of adenosine binding motifs.     

How can natural products serve as a viable source of lead compounds for the development of new/novel anti-malarials?

Malaria continues to be an enormous global health challenge, with millions of new infections and deaths reported annually. This is partly due to the development of resistance by the malaria parasite to the majority of established anti-malarial drugs, a situation that continues to hamper attempts at controlling the disease. This has spurred intensive drug discovery endeavours geared towards identifying novel, highly active anti-malarial drugs, and the identification of quality leads from natural sources would greatly augment these efforts. The current reality is that other than compounds that have their foundation in historic natural products, there are no other compounds in drug discovery as part of lead optimization projects and preclinical development or further that have originated from a natural product start-point in recent years. This paper briefly presents both classical as well as some more modern, but underutilized, approaches that have been applied outside the field of malaria, and which could be considered in enhancing the potential of natural products to provide or inspire the development of anti-malarial lead compounds.     

Anti-malarial activity in a Chinese herbal supplement containing Inonotus obliquus and Panax notoginseng

Plasmodium falciparum, the most virulent human malaria parasite, causes serious diseases among the infected patients in the world and is particularly important in African regions. Although artemisinin combination therapy is recommended by the WHO for treatment of P. falciparum-malaria, the emergence of artemisinin-resistant parasites has become a serious issue which underscores the importance of sustained efforts to obtain novel chemotherapeutic agents against malaria. As a part of such efforts, thirty-nine herbal extracts from traditional Chinese medicine (TCM) were assayed for their anti-malarial activity using 3D7 strain of P. falciparum. Three herbal supplements appeared to possess higher specific anti-malarial activity than the others. One of them (D3) was separated by two sequential fractionations with reverse-phase (the first step) and normal-phase (the second step) liquid chromatography, in which some fractions resulted in higher specific activities than those of D3 or the previous fractions. Cell toxicity assay was performed with the fractions of the first fractionation and demonstrated no obvious cell toxicity. These results suggest that structure determination of the major compound for the anti-malarial activity in D3 may help the development of more potent chemicals in the future.     

A natural product inspired hybrid approach towards the synthesis of novel pentamidine based scaffolds as potential anti-parasitic agents

A natural product inspired molecular hybridization approach led us to a series of novel pentamidine based pyrimidine and chalcone scaffolds. All the hybrids were evaluated for their anti-leishmanial potential. Most of the screened compounds have showed significant in vitro anti-leishmanial activity with less cytotoxicity in comparison to the standard drugs (pentamidine, sodium stibogluconate, and miltefosine). Additionally, anti-malarial screening of these compounds was also done and four compounds have shown superior activity against chloroquine resistance strain (K1) of Plasmodium falciparum.     

Quantum dots: a new tool for anti-malarial drug assays

Background

Malaria infects over 300 million people every year and one of the major obstacles for the eradication of the disease is parasite's resistance to current chemotherapy, thus new drugs are urgently needed. Quantum dot (QD) is a fluorescent nanocrystal that has been in the spotlight as a robust tool for visualization of live cell processes in real time. Here, a simple and efficient method using QD to directly label Plasmodium falciparum-infected erythrocytes (iRBCs) was searched in order to use the QD as a probe in an anti-malarial drug-screening assay.

Methods

A range of QDs with different chemical coatings were tested for their ability to specifically bind iRBCs by immunofluorescence assay (IFA). One QD was selected and used to detect parasite growth and drug sensitivity by flow cytometry.

Results

PEGylated-cationic QD (PCQD) was found to specifically label infected erythrocytes preferentially with late stage parasites. The detection of QD-labelled infected erythrocytes by flow cytometry was sensitive enough to monitor chloroquine anti-malarial toxicity with a drug incubation period as short as 24 h (EC₅₀ = 113nM). A comparison of our assay with another widely used anti-malarial drug screening assay, the pLDH assay, showed that PCQD-based assay had 50% improved sensitivity in detecting drug efficacy within a parasite life cycle. An excellent Z-factor of 0.8 shows that the QD assay is suitable for high-throughput screening.

Conclusions

This new assay can offer a rapid and robust platform to screen novel classes of anti-malarial drugs.

     

Synthesis of a bioprobe for elucidation of target molecule of spongean anti-malarial peroxides

The reactants of an anti-malarial peroxide having a 6-carbomethoxymethyl-3-methoxy-1,2-dioxane moiety treated with FeSO4 were analyzed. For mechanistic study of the anti-malarial peroxide, two biotinylated probes to elucidate the target molecules were designed and synthesized. The two synthesized probes showed potent anti-malarial activity, and one of them was proved to form an irreversible binding with protein in a model experiment.     

New readily accessible peroxides with high anti-malarial potency.

In an explorative study for new anti-malarial substances using the methyl esters (1 and 2) of peroxyplakoric acids A(3) and B(3) as scaffolds, 6-carbomethoxymethyl-3-methoxy-3-pentyl-1,2-dioxane, which has been readily synthesized from 6-keto-alpha,beta-unsaturated ester, was found to exhibit potent anti-malarial activity with high selective toxicity.     

Marine microorganisms as a source of bioactive agents

Several ecological factors of the marine environment were used in developing a strategy for discovering useful bioactive agents from marine microorganisms. By consideration of sea water requirements, production and degradation of marine polymers, and plasmid content, several novel anti-malarial antibiotics, anti-tumor polysaccharides, glucan-degrading enzymes, and aminoglycoside antibiotics were found.     

Site-specific integration and expression of an anti-malarial gene in transgenic Anopheles gambiae significantly reduces Plasmodium infections

Diseases transmitted by mosquitoes have a devastating impact on global health and this is worsening due to difficulties with existing control measures and climate change. Genetically modified mosquitoes that are refractory to disease transmission are seen as having great potential in the delivery of novel control strategies. Historically the genetic modification of insects has relied upon transposable elements which have many limitations despite their successful use. To circumvent these limitations the Streptomyces phage phiC31 integrase system has been successfully adapted for site-specific transgene integration in insects. Here, we present the first site-specific transformation of Anopheles gambiae, the principal vector of human malaria. Mosquitoes were initially engineered to incorporate the phiC31 targeting site at a defined genomic location. A second phase of genetic modification then achieved site-specific integration of Vida3, a synthetic anti-malarial gene. Expression of Vida3, specifically in the midgut of bloodfed females, offered consistent and significant protection against Plasmodium yoelii nigeriensis, reducing average parasite intensity by 85%. Similar protection was observed against Plasmodium falciparum in some experiments, although protection was inconsistent. In the fight against malaria, it is imperative to establish a broad repertoire of both anti-malarial effector genes and tissue-specific promoters for their expression, enabling those offering maximum effect with minimum fitness cost to be identified. In the future, this technology will allow effective comparisons and informed choices to be made, potentially leading to complete transmission blockade.